The strength of unvulcanized rubber is commonly referred to as “green strength”. It is the tensile strength or tensile modulus of an uncured rubber formulation. It is normally quantified in terms of the stress-strain characteristics of the pure rubber or the rubber formulation of interest. Green strength can also be thought of in terms of stress elongation, tensile strength, and creep. ASTM D6746 provides a test method for quantifying green strength. A standard test for measuring green strength is also delineated in the International Standard ISO 9026. In any case, having adequate green strength is critical in the processing of rubber and rubber formulations into useful products. Green strength is the property of a rubbery polymer which allows for it to be built into multiple component articles with little or no release or relative movement of the assembled components subsequent to assembly and prior to initiation of the curing operation.
A high level of green strength is normally desirable to attain good rubber processing behavior. It is a particularly important characteristic for all processing operations in which elongation predominates. For instance, adequate green strength is required for a rubber formulation to perform well in extrusions, calendaring, and tire building operations. In other words, it is important for rubber compounds to have sufficient green strength to be built into rubber composites, such as tires. It is particularly important for the rubber and rubber compounds used in building large tires to have a high level of green strength in order for the rubber layers of the tire to adhere together during the tire building process. It is particularly important in the second stage of building radial tires and in building large tires for trucks, industrial equipment, and earthmovers. In cases where green tires are built with rubber compounds that exhibit poor green strength the tire may fail to hold air during expansion in the second stage of the tire building process prior to cure.
Natural rubber inherently exhibits a relatively high level of green strength. For this reason it is commonly used in building large tires for trucks, industrial equipment, mining equipment, and earthmovers. However, in some applications it would be desirable to further increase the green strength of the natural rubber to more easily facilitate the tire building process. Over the years numerous approaches for increasing the green strength of rubber formulations have been considered. Nevertheless, increasing the green strength of natural rubber in commercial application has proven to be a formidable task.
U.S. Pat. No. 4,094,831 indicates that the green strength of synthetic elastomers can be improved by forming interpolymers from at least one type of various synthetic elastomer forming monomers with an epoxy containing monomer. The elastomer forming monomers employed in the practice of this invention include at least one conjugated diene having from 4 to 10 carbon atoms, olefins having from 2 to 14 carbon atoms, and a diene having from 4 to 6 carbon atoms, and combinations thereof. The interpolymer described by U.S. Pat. No. 4,094,831 are preferably blended with synthetic elastomers or natural rubber (cis-1,4-polyisoprene) and are utilized in various industrial applications, such as in tire carcasses including radial truck tire carcasses. U.S. Pat. No. 4,094,831 indicates that green strength can be further improved if a small amount of an epoxy cross-linking agent is utilized. Examples of suitable cross-linking agents include monoamines and polyamines, monoanhydrides and polyanhydrides, and monocarboxylic acids, as well as polycarboxylic acids.
U.S. Pat. Nos. 4,103,077 and 4,124,750 disclose techniques for improving the green strength of synthetic elastomers by cross-linking them with a dihydrazide compound. U.S. Pat. No. 4,103,077 more specifically reveals a process for improving the green strength of synthetic elastomers, comprising: mixing a synthetic elastomer with a small amount of a dihydrazide compound having the formula NH2—NH—CO—R—CO—NH—NH2, where R is an alkyl group having from 2 to 10 carbon atoms to form a mixture, said synthetic elastomer being made from the solution polymerization of monomers including dienes containing from 4 to 10 carbon atoms, comonomers of dienes containing from 4 to 10 carbon atoms to form copolymers, and comonomers of dienes containing from 4 to 10 carbon atoms with olefin monomers containing from 2 to about 14 carbon atoms to form copolymers; producing an improved green strength elastomer by partially cross-linking said elastomer to effect less than a vulcanized elastomer; and heating said elastomer dihydrazide mixture at a temperature of from about 125° F. (52° C.) to about 300° F. (149° C.). These patents further disclose a synthetic elastomer composition having improved green strength, comprising: a partially cross-linked and non-vulcanized synthetic elastomer; from about 0.25 to about 2.0 parts by weight per 100 parts of said elastomer of a dihydrazide compound having the formula NH2—NH—CO—R—CO—NH—NH2, wherein R is an alkyl group having from 2 to 10 carbon atoms, said synthetic elastomer made from monomers selected from the group consisting of dienes having from 4 to 10 carbon atoms, comonomers of dienes having from 4 to 10 carbon atoms to form copolymers, and comonomers of dienes having from 4 to 10 carbon atoms with olefin monomers having from 2 to about 14 carbon atoms to form copolymers.
U.S. Pat. No. 4,124,546 discloses that the improved green strength of elastomers made from monomers selected from the class consisting of at least one conjugated diene having from 4 to 10 carbon atoms, olefins having from 2 to 14 carbon atoms along with a diene having from 4 to 6 carbon atoms, and combinations thereof can be achieved by adding an amount of a polydimethylbutadiene compound to form a blend having a glass transition temperature of from about 0° C. to about −100° C. The polydimethylbutadiene compound may be merely the homopolymer of dimetylbutadiene, the copolymer, the terpolymer or the tetrapolymer of dimethylbutadiene in various combinations with monomers, such as butadiene, isoprene, piperylene, acrylonitrile, vinylidene chloride, vinyl pyridine, methacrylic acid and vinyl substituted aromatic compounds.
U.S. Pat. Nos. 4,198,324 and 4,243,561 reveal that the green strength of elastomers can be improved by the addition of semi-crystalline butene polymers, such as polybutene and interpolymers made from 1-butene monomer and at least one monomer selected from the class consisting of α-olefins, non-conjugated dienes, and non-conjugated polyenes. The semi-crystalline butene polymer is mixed with a desired elastomer such as natural or synthetic cis-1,4-polyisopropene, or a synthetic elastomer made from monomers selected from the class consisting of conjugated dienes having from 4 to 10 carbon atoms, interpolymers of said dienes among themselves or with vinyl substituted aromatic hydrocarbon compounds having from 8 to 12 carbon atoms, or polyalkenylenes. The mixing or blending of the butene polymer and the elastomer may be through conventional methods such as cement mixing or mastication.
U.S. Pat. No. 4,254,013 indicates that the green strength of elastomer blends of natural or synthetic cis-1,4-polyisoprene and synthetic elastomers can be improved by adding to the chain of the synthetic elastomer an ionogenic compound. The ionogenic compound can be incorporated into the chain of the synthetic elastomer through conventional polymerization with the monomers forming the synthetic elastomer, and the ionogenic group of the compound will be pendant from the chain or backbone of the elastomer. The ionogenic group is combined with a readily ionogenic metal base or salt. This combination yields blends which have greatly improved green strength.